The present invention is directed generally to a cable stay aerodynamic damper band and to its method of use. More specifically, the present invention is directed to a cable stay aerodynamic damper band that is usable with both new cables and as a retro-fit to existing cables. Most particularly, the present invention is directed to damper bands that are securable to cable stays in an application pattern that significantly reduces wind/rain or other induced vibrations in the cable. The cable stay aerodynamic damper bands are structured to be attached to or placed about both new cables as well as existing cables in a particular pattern or array. The use of these damper bands has been very effective in the substantial reduction and near elimination of wind/rain induced vibrations in cable stays and of vibrations induced by the passage of a fluid, such as air or water, over the surface of a cable. The damper bands have an aerodynamic shape that counteracts these vibrations or oscillation induced in the cable stay.
The use of cable stays in the construction of a wide variety of structures is well known. Any number of types of bridges use various cables to support bridge decks, to hold bridge towers steady and to generally form the support for the bridges. Suspension bridges are one example of a bridge structure that uses a large number of elongated cables as stays and supports. In a somewhat similar manner, cables are frequently used as guy wires or as stays in connection with tall antenna towers and the like. A large number of these towers are used to support various receivers, repeaters and other similar assembles. One need not look far without seeing such a tower. A plurality of elongated cables are typically run from various elevations on these towers to suitable ground anchors. These cable stays or guy wires are used to stabilize the tower.
Elongated cables are also utilized in the underwater stabilization of floating oil drilling installations. These cables are subjected to hydrodynamic forces that are very similar to the aerodynamic forces which above ground stay cables and guy lines experience.
In all of these cable applications, the passage of a fluid, such as air or water or of wind-driven rain, over the surface of the cable induces vibration or oscillation in the cable. If the fluid velocity is sufficient, the cable can be seen to vibrate at node points with sufficient amplitude that the structure with which the cable is associated may be damaged. In the case of bridge cable stays, the bridge stays may be caused to vibrate or in extreme situations to shake sufficiently that the structural integrity of the bridge may be compromised. Such vibrations can also cause fatigue in the cables. In the situation involving sub-sea cables, the position of the anchored platform can be affected with a resultant possible mis-alignment of platform supported drill strings and other similar downhole implements.
It has been proposed in the past to provide various mechanical vibration dampers for elongated cables. In one configuration, these vibration dampeners have taken the form of shock-absorber like devices that may be interposed between an end of the cable and an anchoring or attachment site for the cable. Other similar spring-biased connections have been used in the past in an effort to compensate for or to counteract wind/rain or high speed wind induced vibrations and oscillations.
Fairings and streamlining devices have, in the past, been applied to overhead cables, to sub-sea cables and to guy wires and cable stays. These attempt to altar the shape of the generally cylindrical cable to create an airfoil or flow-smoothing shape.
It is also known in the art to fabricate structures with integrally formed annular rings and with various projections and protrusions. In these structures, the rings are formed during the fabrication of the structure, which may be a mast of an outdoor antenna, a smokestack, transmission lines or pipelines. These rings are intended to reduce or to eliminate the vortex shedding which affects structures of these types. The elimination of this vortex shedding will greatly reduce the oscillating lateral forces which smokestacks, antennas, transmission lines and other cylindrical structure have been plagued by due to this periodic shedding of vortices.
While the prior art has appreciated the use of various vibration dampers and integrally formed annular rings and bands as well as various fairings and spoilers, there continues to exist a need for cable stay aerodynamic dampers and their method of use and application which will overcome the limitations of the prior art devices.
It is an object of the present invention to provide a cable stay aerodynamic damper band.
Another object of the present invention is to provide a method of using cable stay aerodynamic damper bands.
A further object of the present invention is to provide a cable stay aerodynamic damper band for retrofit use.
Still another object of the present invention is to provide a cable stay aerodynamic damper band which is effective in counteracting wind and rain induced vibration.
Yet a further object of the present invention is to provide a cable stay aerodynamic damper band which damps both low and high modal vibrations.
Even still another object of the present invention is to provide a cable stay aerodynamic damper band which is economical to use and which is easily attached.
As will be set forth in greater detail in the description of the preferred embodiments which are presented subsequently, the cable stay aerodynamic damper band in accordance with the present invention, and its method of use is primarily intended to counteract wind and rain induced vibrations in cable stays of structures, such as bridges and the like. It has been determined that the presence of small livers or rivulets of water running along the length of a cable stay, in combination with wind velocities in the range of 15-35 MPH can create very dramatic vibrations in the cable stay. These vibrations are not always the high modal vibrations such as would occur with high wind velocity directed perpendicularly to a taut cable or cylindrical structure and which typically create classic vortex shedding. Instead, these vibrations, which typically occur when the wind is coming from behind the cable, will typically cause first, second and third modes of vibrations. These wind/rain induced vibrations are clearly visible to the naked eye and thus are very unsettling to a motorist traveling across the bridge supported by these vibrating cable stays. More importantly, these low modal, violent vibrations can and do cause significant cable fatigue and other structural problems.
The existence of wind/rain induced vibrations in cable stays is a phenomenon that can be counteracted by properly designing the stay cables of a structure before it is erected. Unfortunately, there has not, in the prior art, been a practical retro-fit solution for bridges and cable stays which are already in place. The use of mechanical dampers at both ends of the cable is one current solution. Such mechanical dampers act as shock absorbers. They do nothing to prevent the wind/rain induced vibrations of the cable. They merely attempt to prevent it from being transmitted to the bridge structure. These mechanical dampers are large, expensive, heavy devices which are difficult to install and which have only a marginal amount of success.
Another current solution is to utilize a restrainer system in which adjacent cable stays are connected to each other by fixed length bars or stabilizers. This solution is again difficult and costly to implement, and may give rise to induced vibrations in adjacent cables. Further, the use of these restraint systems generally destroys the aesthetics of the bridge design.
The cable stay aerodynamic dampers bands, and their method of usage, in accordance with the present invention, provide an effective, cost efficient solution to the problem of cable stay vibration and particularly to wind/rain induced cable stay vibration. The cable stay damper bands utilize flexible or rigid cable encircling bands which carry embedded or attached tension straps. The cable encircling bands can be placed about existing cables in the field without taking the structure, such as a bridge out of service and without the need for large amounts of specialized equipment.
The cable stay aerodynamic damper bands of the present invention break up the rivulets or small rivers of water which tend to form on, and to travel along the cable stays. It is believed that these rivulets tend to act as an airfoil on the cable stays and that their existence is quite important to the generation of the low modal cable stay vibrations which the damper bands of the present invention have been so effective in reducing or eliminating. The damper bands of the present invention will also be effective in reducing or eliminating vortex shedding induced vibrations, such as the so called KARMAR VORTEX STREET vibrations. However, such higher mode vibrations typically 7th or 8th mode vibrations, are not as severe with respect to fatigue loadings as the lower mode wind/rain induced vibrations whose elimination is the primary problem to which the present cable stay aerodynamic damper bands are directed.
The damper bands of the present invention have been found to be very effective when applied to an existing cable stay in a pattern of bands placed along the cable at a spacing of preferably twice to four times the diameter of the cable. While this will result in the use of a large number of bands, the number of these bands is nowhere near the number suggested in the prior art as being required. The flexible or rigid bands can be installed effectively using uncomplicated techniques so that minimal disruption to the structure during damper band installation will occur.
Unlike prior proposed solutions, the cable stay aerodynamic damper bands of the present invention are not particularly conspicuous, do not require adjacent cables to be connected together, are durable and require essentially no maintenance, and are not apt to add a great amount of weight to the cable stays to which they are attached. The cable stay aerodynamic damper bands and their methods of use, in accordance with the present invention, overcome the limitations of the prior art solutions. They represent a substantial advance in the art.